I experience depression as an autonomous shutdown, under great mental weight.

The ability to make any mental and physical effort is greatly reduced to "depressed" levels.

It is the subconscious version of a situation where you need to climb out of a pit, but the walls are perfectly flat and oiled. Even though you need to get out of the pit, instead of scrambling, you just sit down.

The situation may not really be like that, but some deep cognitive machinery has concluded that it is.

It isn't anything like sadness. Although sadness could be part or proximate cause of a particular depressive episode, it is neither a necessary or sufficient condition.

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I do believe depression has a real purpose. To turn off motivated but misdirected behavior, and force us to perform a reset of deeply held assumptions, expectations and motivations.

Pain effectively changes our expectations and behavior in the moment. Depression is like pain for complex situations. It forces major change, by disabling our current ineffective directives and giving us "time out" to slowly rewire. Until the subconscious believes we have found a new mental path, more aligned with reality, and lets motivation flow again.

But like battle surgery, it is a blunt instrument for doing something profound, and may inflict damage as well as repair it.

And when life circumstances don't let us take the pause and rest our brain is trying to enforce, it is a bit of living hell.